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Looking for Technics SU-2500 amplifier schematics

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Member

Hi, is there anyone that could send me the schematics (or service manual) for a Technics-SU2500 amplifier? I'm repairing one of these vintage amplifiers, and I didn't succeed to find the manual or schematic anywhere. Thanks in advance!

chances are you can measure the DC power supply voltage between one of the pins of the bigger 0.33 Ohm emitter resistors (the thick green ones close to the first pins of the STK) and the audio ground.
If so replace the STK.

People have a tendency to overestimate their hearing capabilities.
Use your ears to listen to music not as an analyzer.

Member

Hi,
First of all I wanted to tell you that this amplifier was found abandoned and I don't know about its history or the old owner treatments. It was not working, but VU-meter lamps were lighting. I was checking a lot of things in order to know what was going wrong. First of all there is a DC output from both speakers connections (about DC 26v is liking from left and right connectors). I was lucky to take decision for not connecting any speaker on them!
Between the left side or R219 and pin1 of STK0039 (left channel) there are +1,1v, and in the right (R220) side -1,1V. Here is a table showing the DC volts from every STK pin and the ground:

For curiosity and in order to try I replaced both C1885 transistors for C2705 (more easily available), and both Integrated A798 for two couples of BC557 with a similar hFE.
These replacements are still standing on the circuit.

Founder/Admin

Set your multimeter to diode setting (or ohm). Check every transistor and see if any of them show a short from collector to emitter. And from base to either collector or emitter. You should also get 0.6 volt between base and emitter (in one direction only).

Member

Hi, after replacing the fuse resistor R238, the right channel nicely started to work! and right channel speaker connectors are liking DC: 0,0V
At this point it seems half of the problem was fixed.

The output from the left side is liking now DC: -17,6V (there was a change in the polarity)
I have checked the R261 and R262 green resistors and they appeared to be in a good condition: Nominal value for them is 6,8ohm, and the measured value was 7,2ohm for both of them.
This is the new table of volts I obtained by measuring the stk0039 pins:

PIN1: 19,1 / -1,2 (VOLTS Left / VOLTS Right channel)
PIN2: 35,2 / -35,1
PIN3: -17,4 / 0
PIN4: 0 / 0
PIN5: -33,9 / -34,8
PIN6: 0 / 0
PIN7: 0 / 0
PIN8: -17,8 / 0
PIN9: 35,4 / 35,5
PIN 0: -16,2 / 1,3
As you can see the values for the right channel (actually working) turned to be close values, as written in the Solderdude schematic.
I checked the resistances R249, and R251, and they are fine (I don't understant from where are coming the -16v negative in pin 0 of STK).
I don't know where is the problem with left channel now
Thanks for your help

Major Contributor

You need to check all the fusible resistors against their marked value (the colour codes), including the ones in the channel we have 'fixed'. Lift one end of each fusible resistor and measure the resistance. The 1k and 2.7K are suspect.

To give you an idea, here is the fusible resistor complement from a little Harman Kardon PM635i integrated amplifier from a similar era to your Technics. Consider the amplifier was 'working' but exhibiting poor performance...

As you can see, there was 1 (one) only that was within tolerance (the one with my tick next to it). The rest were so far out of tolerance, I made this image to help people understand how far the little devils can drift in their old age.

Ricky Roma: I subscribe to the law of contrary public opinion... If everyone thinks one thing, then I say, bet the other way...

Major Contributor

Further, consider your pin 1 on the STK is sitting at 19.1V. Look at the schematic below of the darlington pack:

Pin 1 will be naturally pulled high via (the internal to the STK) R1 and R2 from the + supply in this Technics implementation, unless it is pulled down via TR203/5 (on the PCB) and the two fusibles I mentioned above.

Ricky Roma: I subscribe to the law of contrary public opinion... If everyone thinks one thing, then I say, bet the other way...

Member

Dear friends, I have just replaced the pre-driver transistor (2sc2705) for a 2sc3619, and ended all the DC current from the left side speaker connectors. After to test the old 2705 I saw there was some fault on it. Now the left side is also working!!!
At the moment (and temporarily) I'm using a 2sc2705 in the right channel, and a 2SC3619 in the left channel as pre-driver transistors. Both of them are replacing the original and hard to find 2SC1885. I'd appreciate to know what is for this circuit, the best option for the replacement of both 2SC1885, in order to purchase 2 new modern transistors and to have identical composition in both channels.
restorer-john, thanks a lot for your observations about the fuses and for your patience.
thanks

Major Contributor

2SC-1885 is an original Hitachi transistor, but Matsushita also made it. It's likely a T092L (for long).

You can use a Sanyo 2SC-2999, an NEC 2SC-1393, a Matsushita 2SC-1547 or a Mitsubishi 2SC-738, they are all essentially identical. Each transistor is obsolete in real terms, but you should be able to find a genuine pair if you hunt around. They have used very fast (RF) transistors.

Member

Hi, finally I found the 2SC-2999 in a reasonable price, so I'm waiting to get it from UK.
After listening one couple of songs, the right channel went wrong again, and now returned back the 36,3V DC output. I was checking all fuses and most of the caps. When happened this problem R230 was burning (it is still fine), and I needed to replace TR202 by a new couple paired BC557 transistors. R236, R230, 2228, c218 and c224 are fine. The volts of TR202 are fine. Here is the neu table of volts I obtained by measuring the right side stk0039 pins:
PIN1: 35,5
PIN2: 35,4
PIN3: 36,2
PIN4: 0
PIN5: 36
PIN6: 0
PIN7: 0
PIN8: 36,1
PIN9: 36,1
PIN 0: 33
I star to think that in STK0039 there is a short and it is dead. What do you think about this?
Thanks a lot for your help

Major Contributor

Switch off the amp... let the capacitors bleed for a while.
Set your MM to Ohms or diode setting and measure between pin 8 and pin 9 in both directions (so red on 8 and black on 9 and once with black on red and red on 9)
When it reads close to 0 Ohm in both directions the device is broken.

People have a tendency to overestimate their hearing capabilities.
Use your ears to listen to music not as an analyzer.

Member

Between pin 8 and 9 and opposite, the measurement is 2,2 ohm; the measurement between these pins in the good STK is about 10K ohm in both directions (not stable measurement). So I assume the right STK0039 is dead. I understand there are a lot of fake STK in the market and it is very difficult to find a working one. Is there any recommendation, about purchasing a good and not so expensive stk, to take into account before selecting one of them from internet?

I attached a fck STK, that I ordered from China some years ago, and after to see that I have a nice paperweight, I decided to open...
Thanks again